Zone boundary adjustments for defects in non-volatile memories

ABSTRACT

A non-volatile memory is divided into logical zones by the card controller in order reduce the size of the data structures it uses for address translation. Zone boundaries are adjusted to accommodate defects allowed by memory test to improve card yields and to adjust boundaries in the field to extend the usable lifetime of the card. Firmware scans for the presence of defective blocks on the card. Once the locations of these blocks are known, the firmware calculates the zone boundaries in such a way that good blocks are equally distributed among the zones. Since the number of good blocks meets the card test criteria by the memory test criteria, defects will reduce card yield fallout. The controller can perform dynamic boundary adjustments. When defects occur, the controller can perform the analysis again and, if needed, redistributes the zone boundaries, moving any user data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/114,996,filed on Apr. 26, 2005, which is a continuation of application Ser. No.10/315,451, filed on Dec. 9, 2002. These applications are allincorporated herein by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

This invention pertains to the field of semiconductor non-volatile datastorage system architectures and their methods of operation, and hasapplication to data storage systems based on flash electrically erasableand programmable read-only memories (EEPROMs).

A common application of flash EEPROM devices is as a mass data storagesubsystem for electronic devices. Such subsystems are commonlyimplemented as either removable memory cards that can be inserted intomultiple host systems or as non-removable embedded storage within thehost system. In both implementations, the subsystem includes one or moreflash devices and often a subsystem controller.

Flash EEPROM devices are composed of one or more arrays of transistorcells, each cell capable of non-volatile storage of one or more bits ofdata. Thus flash memory does not require power to retain the dataprogrammed therein. Once programmed however, a cell must be erasedbefore it can be reprogrammed with a new data value. These arrays ofcells are partitioned into groups to provide for efficientimplementation of read, program and erase functions. A typical flashmemory architecture for mass storage arranges large groups of cells intoerasable blocks, wherein a block contains the smallest number of cells(unit of erase) that are erasable at one time.

In one commercial form, each block contains enough cells to store onesector of user data plus some overhead data related to the user dataand/or to the block in which it is stored. The amount of user dataincluded in a sector is the standard 512 bytes in one class of suchmemory systems but can be of some other size. Because the isolation ofindividual blocks of cells from one another that is required to makethem individually erasable takes space on the integrated circuit chip,another class of flash memories makes the blocks significantly larger sothere is less space required for such isolation. But since it is alsodesired to handle user data in much smaller sectors, each large block isoften further partitioned into individually addressable pages that arethe basic unit for reading and programming user data. Each page usuallystores one sector of user data, but a page may store a partial sector ormultiple sectors. A “sector” is used herein to refer to an amount ofuser data that is transferred to and from the host as a unit.

The subsystem controller in a large block system performs a number offunctions including the translation between logical addresses (LBAs)received by the memory sub-system from a host, and physical blocknumbers (PBNs) and page addresses within the memory cell array. Thistranslation often involves use of intermediate terms for a logical blocknumber (LBN) and logical page. The controller also manages the low levelflash circuit operation through a series of commands that it issues tothe flash memory devices via an interface bus. Another function thecontroller performs is to maintain the integrity of data stored to thesubsystem through various means, such as by using an error correctioncode (ECC).

FIG. 1 shows a typical internal architecture for a flash memory device131. The primary features include an input/output (I/O) bus 411 andcontrol signals 412 to interface to an external controller, a memorycontrol circuit 450 to control internal memory operations with registersfor command, address and status signals. One or more arrays 400 of flashEEPROM cells are included, each array having its own row decoder (XDEC)401 and column decoder (YDEC) 402, a group of sense amplifiers andprogram control circuitry (SA/PROG) 454 and a data register 404.Presently, the memory cells usually include one or more conductivefloating gates as storage elements but other long term electron chargestorage elements may be used instead. The memory cell array may beoperated with two levels of charge defined for each storage element totherefore store one bit of data with each element. Alternatively, morethan two storage states may be defined for each storage element, inwhich case more than one bit of data is stored in each element.

If desired, a plurality of arrays 400, together with related X decoders,Y decoders, program/verified circuitry, data registers, and the like areprovided, for example as taught by U.S. Pat. No. 5,890,192, issued Mar.30, 1999, and assigned to SanDisk Corporation, the assignee of thisapplication, which is hereby incorporated by this reference. Relatedmemory system features are described in co-pending patent applicationSer. No. 09/505,555, filed Feb. 17, 2000 by Kevin Conley et al., whichapplication is expressly incorporated herein by this reference.

The external interface I/O bus 411 and control signals 412 can includethe following:

-   -   CS—Chip Select. Used to activate flash memory interface.    -   RS—Read Strobe. Used to indicate the I/O bus is being used to        transfer data from the memory array.    -   WS—Write Strobe. Used to indicate the I/O bus is being used to        transfer data to the memory array.    -   AS—Address Strobe. Indicates that the I/O bus is being used to        transfer address information.    -   AD[7:0]—Address/Data Bus This I/O bus is used to transfer data        between controller and the flash memory command, address and        data registers of the memory control 450.

In addition to these signals, it is also typical that the memory has ameans by which the storage subsystem controller may determine that thememory is busy performing some task. Such means could include adedicated signal or a status bit in an internal memory register that isaccessible while the memory is busy.

This interface is given only as an example as other signalconfigurations can be used to give the same functionality. FIG. 1 showsonly one flash memory array 400 with its related components, but amultiplicity of such arrays can exist on a single flash memory chip thatshare a common interface and memory control circuitry but have separateXDEC, YDEC, SA/PROG and DATA REG circuitry in order to allow parallelread and program operations.

Data is transferred from the memory array through the data register 404to an external controller via the data registers' coupling to the I/Obus AD[7:0] 411. The data register 404 is also coupled the senseamplifier/programming circuit 454. The number of elements of the dataregister coupled to each sense amplifier/programming circuit element maydepend on the number of bits stored in each storage element of thememory cells, flash EEPROM cells each containing one or more floatinggates as the storage elements. Each storage element may store aplurality of bits, such as 2 or 4, if the memory cells are operated in amulti-state mode. Alternatively, the memory cells may be operated in abinary mode to store one bit of data per storage element.

The row decoder 401 decodes row addresses for the array 400 in order toselect the physical page to be accessed. The row decoder 401 receivesrow addresses via internal row address lines 419 from the memory controllogic 450. A column decoder 402 receives column addresses via internalcolumn address lines 429 from the memory control logic 450.

FIG. 2 shows an architecture of a typical non-volatile data storagesystem, in this case employing flash memory cells as the storage media.In one form, this system is encapsulated within a removable card havingan electrical connector extending along one side to provide the hostinterface when inserted into a receptacle of a host. Alternatively, thesystem of FIG. 2 may be embedded into a host system in the form of apermanently installed embedded circuit or otherwise. The system utilizesa single controller 101 that performs high-level host and memory controlfunctions. The flash memory media is composed of one or more flashmemory devices, each such device often formed on its own integratedcircuit chip. The system controller and the flash memory are connectedby a bus 121 that allows the controller 101 to load command, address,and transfer data to and from the flash memory array. (The bus 121includes 412 and 411 of FIG. 1.) The controller 101 interfaces with ahost system (not shown) with which user data is transferred to and fromthe flash memory array. In the case where the system of FIG. 2 isincluded in a card, the host interface includes a mating plug and socketassembly (not shown) on the card and host equipment.

The controller 101 receives a command from the host to read or write oneor more sectors of user data starting at a particular logical address.This address may or may not align with the first physical page in ablock of memory cells.

In some prior art systems having large capacity memory cell blocks thatare divided into multiple pages, the data from a block that is not beingupdated needs to be copied from the original block to a new block thatalso contains the new, updated data being written by the host. In otherprior art systems, flags are recorded with the user data in pages andare used to indicate that pages of data in the original block that arebeing superceded by the newly written data are invalid. A mechanism bywhich data that partially supercedes data stored in an existing blockcan be written without either copying unchanged data from the existingblock or programming flags to pages that have been previously programmedis described in co-pending patent application “Partial Block DataProgramming and Reading Operations in a Non-Volatile Memory”, Ser. No.09/766,436, filed Jan. 19, 2001 by Kevin Conley, which application isexpressly incorporated herein by this reference.

Non-volatile memory systems of this type are being applied to a numberof applications, particularly when packaged in an enclosed card that isremovable connected with a host system. Current commercial memory cardformats include that of the Personal Computer Memory Card InternationalAssociation (PCMCIA), CompactFlash (CF), MultiMediaCard (MMC) and SecureDigital (SD). One supplier of these cards is SanDisk Corporation,assignee of this application. Host systems with which such cards areused include personal computers, notebook computers, hand-held computingdevices, cameras, audio reproducing devices, and the like. Flash EEPROMsystems are also utilized as bulk mass storage embedded in host systems.

Such non-volatile memory systems include one or more arrays offloating-gate memory cells and a system controller. The controllermanages communication with the host system and operation of the memorycell array to store and retrieve user data. The memory cells are groupedtogether into blocks of cells, a block of cells being the smallestgrouping of cells that are simultaneously erasable. Prior to writingdata into one or more blocks of cells, those blocks of cells are erased.User data are typically transferred between the host and memory array insectors. A sector of user data can be any amount that is convenient tohandle, preferably less than the capacity of the memory block, oftenbeing equal to the standard disk drive sector size, 512 bytes. In onecommercial architecture, the memory system block is sized to store onesector of user data plus overhead data, the overhead data includinginformation such as an error correction code (ECC) for the user datastored in the block, a history of use of the block, defects and otherphysical information of the memory cell block. Various implementationsof this type of non-volatile memory system are described in thefollowing United States patents and pending applications assigned toSanDisk Corporation, each of which is incorporated herein in itsentirety by this reference: U.S. Pat. Nos. 5,172,338, 5,602,987,5,315,541, 5,200,959, 5,270,979, 5,428,621, 5,663,901, 5,532,962,5,430,859 and 5,712,180, and application Ser. No. 08/910,947, filed Aug.7, 1997, and application Ser. No. 09/343,328, filed Jun. 30, 1999.Another type of non-volatile memory system utilizes a larger memory cellblock size that stores multiple sectors of user data.

One architecture of the memory cell array conveniently forms a blockfrom one or two rows of memory cells that are within a sub-array orother unit of cells and which share a common erase gate. U.S. Pat. Nos.5,677,872 and 5,712,179 of SanDisk Corporation, which are incorporatedherein in their entirety, give examples of this architecture. Althoughit is currently most common to store one bit of data in each floatinggate cell by defining only two programmed threshold levels, the trend isto store more than one bit of data in each cell by establishing morethan two floating-gate transistor threshold ranges. A memory system thatstores two bits of data per floating gate (four threshold level rangesor states) is currently available, with three bits per cell (eightthreshold level ranges or states) and four bits per cell (sixteenthreshold level ranges) being contemplated for future systems. Ofcourse, the number of memory cells required to store a sector of datagoes down as the number of bits stored in each cell goes up. This trend,combined with a scaling of the array resulting from improvements in cellstructure and general semiconductor processing, makes it practical toform a memory cell block in a segmented portion of a row of cells. Theblock structure can also be formed to enable selection of operation ofeach of the memory cells in two states (one data bit per cell) or insome multiple such as four states (two data bits per cell), as describedin SanDisk Corporation U.S. Pat. No. 5,930,167, which is incorporatedherein in its entirety by this reference.

Since the programming of data into floating-gate memory cells can takesignificant amounts of time, a large number of memory cells in a row aretypically programmed at the same time. But increases in this parallelismcause increased power requirements and potential disturbances of chargesof adjacent cells or interaction between them. U.S. Pat. No. 5,890,192of SanDisk Corporation, which is incorporated above, describes a systemthat minimizes these effects by simultaneously programming multiplepages (referred to as chunks in that patent) of data into differentblocks of cells located in different operational memory cell units(sub-arrays). Memory systems capable of programming multiple pages inparallel into multiple sub-array units are described in co-pendingpatent application Ser. No. 09/505,555, filed Feb. 17, 2000 by KevinConley et al., which is incorporated by reference above, and Ser. No.09/759,835, filed Jan. 10, 2001, by John Mangan et al., whichapplication is expressly incorporated herein by this reference.

More detail on a specific embodiment of FIG. 2 is shown in FIG. 3. Thisparticular embodiment divides the memory array 400 into a number of“planes”, where a plane is a subdivision of the memory on a single die.Only the more relevant portions of FIG. 3 will be described here. Moredetail can be found U.S. patent application Ser. No. 09/759,835 that wasincorporated by reference in the previous paragraph.

The non-volatile memory chip 17 includes a logic circuit 39 forinterfacing with the controller through the lines 302. Additionalcomponents of the memory chip are not shown for simplicity inexplanation. The purpose of the logic circuit 39 is to generate signalsin separate buses and control lines. Various control signals areprovided in lines 41 and a power supply 43 to the memory array circuitsis also controlled through the interface 39. A data bus 45 carries userdata being programmed into or read from the non-volatile memory, and anaddress bus 47 carries the addresses of the portion of the memory beingaccessed for reading user data, writing user data, or erasing blocks ofmemory cells.

The floating gate memory cell array of a single non-volatile memory chipis itself divided into a number of units that each have its own set ofsupporting circuits for addressing, decoding, reading and the like. Inthis example, eight such array units 0-7, denoted by reference numbers51-58, are illustrated. Physically, as an example, the memory array on asingle chip is divided into quadrants, or “planes”, each quadrantincluding two units that are in part connected together and share acommon word line decoding circuits (y-decode), such as the y-decoders 61and 62 on either side of memory cell units 4 (55) and 5 (56). The commonword lines run across both memory cell units 4 (55) and 5 (56), withhalf connected to the y-decoder 61 on one side and half connected toy-decoder 62 on the other side, as described further below, with respectto FIG. 3. This memory architecture is similar to that described in U.S.Pat. No. 5,890,192 incorporated by reference above, except there areeight units, or “planes”, instead of the four units (quads) illustratedin that patent.

A number of architectures are used for non-volatile memories arrays,such as 400 (FIG. 1) or 51-58 (FIG. 3). A NOR array of one design hasits memory cells connected between adjacent bit (column) lines andcontrol gates connected to word (row) lines. The individual cellscontain either one floating gate transistor, with or without a selecttransistor formed in series with it, or two floating gate transistorsseparated by a single select transistor. Examples of such arrays andtheir use in storage systems are given in the following U.S. patents andpending applications of SanDisk Corporation that are incorporated hereinin their entirety by this reference or which have been previouslyincorporated above: U.S. Pat. Nos. 5,095,344, 5,172,338, 5,602,987,5,663,901, 5,430,859, 5,657,332, 5,712,180, 5,890,192, and 6,151,248,and Ser. No. 09/505,555, filed Feb. 17, 2000, and Ser. No. 09/667,344,filed Sep. 22, 2000.

A NAND array of one design has a number of memory cells, such as 8, 16or even 32, connected in series string between a bit line and areference potential through select transistors at either end. Word linesare connected with control gates of cells in different series strings.Relevant examples of such arrays and their operation are given in thefollowing U.S. patent application Ser. No. 09/893,277, filed Jun. 27,2001, that is also hereby incorporated by reference, and referencescontained therein.

A memory will often have defective portions, either from themanufacturing process or that arise during the operation of the device.A number of techniques exist for managing these defects, such as witherror correction code or by remapping portions of the memory, such asdescribed in U.S. Pat. No. 5,602,987, that was incorporated by referenceabove, or U.S. Pat. Nos. 5,315,541, 5,200,959, and 5,428,621, that arehereby incorporated by reference. For instance, a device is generallythoroughly tested before being shipped. The testing may find a defectiveportion of the memory that needs to be eliminated. Before shipping thedevice, the information on these defects can be stored on the device,for example in a ROM area of the memory array or separate ROM, and atpower up it is read by controller and then used so that the controllercan substitute a good portion of the memory for the bad. When reading orwriting, the controller will then need to refer to a pointer structurein controller for this re-mapping.

Memories are often designed with a number of redundant blocks to replacedefective blocks. These are generally distributed between the logicalstructure areas of the memory. However, if the number of bad blocks istoo large, or too unevenly distributed, this results in yield loses ordowngrades the capacity of the card.

SUMMARY OF THE INVENTION

According to one principal aspect of the present invention, briefly andgenerally, a non-volatile memory, such as a flash memory, is dividedinto logical zones by the card controller in order reduce the size ofthe data structures it uses for address translation. The presentinvention presents methods to adjust the zone boundaries to accommodatedefects allowed by memory test to improve card yields and to adjustboundaries in the field to extend the usable lifetime of the card. Atthe time of card production, an operation is performed that sets zoneboundaries. During this process, the firmware scans for the presence ofdefective blocks on the card. Once the locations of all the blocks areknown, the firmware calculates the zone boundaries in such a way thatgood blocks are equally distributed among the zones. Since the number ofgood blocks meets the card test criteria by the memory test criteria,defects will reduce card yield fallout.

In another aspect of the present invention, the controller can performdynamic boundary adjustments. When defects occur, the controllerperforms the analysis again and, if the number of good blocks becomesunbalanced, redistributes the zone boundaries, moving any user data. Theprocess can be reformed based on a timing mechanism, such as the numberof program or erase cycles, in response to an error correction code(ECC) result, or an indication of programming or erase difficulties,such as verification failure.

Additional aspects, features and advantages of the present invention areincluded in the following description of exemplary embodiments, whichdescription should be read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical prior art flash EEPROM memoryarray with memory control logic, data and address registers.

FIG. 2 illustrates an architecture utilizing memories of FIG. 1 with asystem controller.

FIG. 3 illustrates some elements of a non-volatile memory.

FIG. 4 illustrates an arrangement of bad blocks in a single planeembodiment.

FIG. 5 shows an example of boundary adjustment in a four plane, ten zoneembodiment.

FIG. 6 is a schematic illustration of how boundaries can be realigned.

FIG. 7 is a flow chart illustrating an exemplary embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

The various aspects of the present invention are applicable tonon-volatile memory systems in general. Although the description below,as well as that in the Background, is given mainly in terms of an EEPROMFlash memory embodiment, the particular type of storage unit used in thememory array is not particularly important in the present invention. Theparticulars of how the storage elements are read, are written, and storedata do not enter in to the main aspects of the present invention andcan be those of any of the various non-volatile systems.

In the following, as in the Background section, the followingterminology is used: a block contains the smallest number of cells (unitof erase) that are erasable at one time, a sector is used herein torefer to an amount of user data that is transferred to and from the hostas a unit, a page is the basic unit for reading and programming userdata, and a plane is physical subdivision of the memory on a single die.Although in some embodiments, some of these structures correspond,either directly or through a logical to physical correspondence, to oneanother, they are conceptually distinct. Additionally, in the moregeneral case, a read page and a write page may be distinct. In thepresent discussion, the additional term a “zone” is used for a logicalsubdivision of the total capacity of the die. In a main aspect of thecurrent invention, logically an address is distributed over the planesbut the physical boundaries of a zone within a plane are independent andare optimized for the location of defects.

In the exemplary embodiment of the present invention, a flash or othermemory device, such as 17 in FIG. 3, is divided into logical zones bythe card controller 301 in order reduce the size of the data structuresit uses for address translation. Within a zone, a certain number ofphysical blocks are often included beyond the logical capacity of thezone in order to provide an erase pool used for write operations. Sincethe logical zones and their defect tolerances may not align to thenumber of defects in each zone found during memory testing, it ispossible to suffer yield loss due to having excessive defects in a zone.In addition, defects can occur during operation of the card that cancause a zone to be unusable thus limiting the lifetime of the card. Aprinciple aspect of the present invention is to adjust the zoneboundaries to accommodate defects allowed by memory test to improve cardyields and to adjust boundaries in the field to extend the usablelifetime of the card.

The situation is illustrated schematically in FIG. 4 for a memory array400 having a single plan that is logically divided into six zones,indicated by broken lines, plus a number of system blocks devoted tooverhead and other non-user data. A number of bad blocks, 511-523, areillustrated by a blackened strip. For example, bad block 511 in ZONE0and bad block 513 in ZONE3 can be a pair of blocks that were acceptablewhen the device was initially tested, but which failed as the device wasused; while bad blocks 515, 517, 519, and 521 in ZONE4 and bad block 523in ZONE5 can be blocks that were bad at initial testing due to, say,processing errors during manufacture. If each zone is allotted threespare blocks, ZONE4 will not have enough good blocks to correspond toits logical assignments.

One approach to this problem is to provide more spare blocks, but thisis a trade off between wasted space (and consequently cost) versusreliability. Although it is standard practice to include a number ofspare or redundant blocks in an array, there is usually a limit beyondwhere this becomes counterproductive. Furthermore, when an array islogically divided into sub-units, such as the zone structure describedhere, the spare blocks are typically distributed uniformly across thesub-units. However, the defective blocks may be clustered in a givensub-unit, either through randomness or due to processing or otherproblems that affect a limited area. Consequently, the number of spareblocks may be spread too thin in one zone while another zone maintainsits original excess number of spare blocks.

A previous approach to this problem involved assigning boundaries at thesame time as the memory testing, for example moving a boundary for ZONE4to add the need extra good block. This limits flexibility in reducingzones to better meet cost requirements of controller memory. Anotherapproach is to suffer yield loses or downgrade the capacity of the card.

In a principle aspect of the present invention, the controller scans forthe presence of defective blocks on the card. This can be done at thetime of card production with an operation performed that sets zoneboundaries. In an exemplary embodiment, this is implemented in firmware,although hardware and software implementations are variations. Once thelocations of all the blocks are known, the firmware calculates the zoneboundaries in such a way that good blocks are equally distributed amongthe zones. Since the number of good blocks is guaranteed to meet thecard test criteria by the memory test criteria, defects will cause nocard yield fallout. When defects occur, the controller performs theanalysis again and, if the number of good blocks becomes unbalanced,redistributes the zone boundaries, moving any user data.

For example, in FIG. 4, at test time, the boundary between ZONE4 andZONE3 can be moved to provide ZONE4 with additional non-defectiveblocks. If bad block 513 or additional bad blocks in ZONE3 develop overtime, the controller can subsequently move up the ZONE3-ZONE2 boundaryto insure sufficient good blocks in the array.

Since each zone is typically provided with enough spare blocks to allowa reasonable number of defective blocks, when the array is contained ina single plain there is usually enough reserve so that boundaries willgenerally not require much movement. However, when the zones aredistributed across several planes, these margins can become spread quitethin. For example, if each zone is allotted eight extra blocks, asingle-plane embodiment as in FIG. 4 would allow up to all eightdefective blocks in the zone before the zone becomes unusable withboundary adjustment. If instead, each zone is distributed across fourplanes, that portion of each zone in a particular plane can tolerate atmost two defects.

The use of planes is discussed above in the Background. In order toimprove performance by reducing programming time, a goal is to programas many cells in parallel as can reasonably be done without incurringother penalties. One implementation is the use of planes that divide thememory array into largely independent sub-arrays or units, each unit inturn being divided into a large number of blocks, as described in U.S.patent application Ser. No. 09/505,555, filed Feb. 17, 2000, by KevinConley et al. and Ser. No. 09/759,835, filed Jan. 10, 2001, by JohnMangan et al., which are incorporated by reference above. Pages of dataare then programmed at the same time into more than one of the units.Another configuration further combines one or more of these units frommultiple memory chips. These multiple chips may be connected to a singlebus (as shown in FIG. 2) or multiple independent busses for higher datathroughput.

An extension of this is to link blocks from different units forprogramming, reading and erasing together, as described in U.S. patentapplication Ser. No. 09/766,436, filed Jan. 19, 2001, which isincorporated by reference above. In this arrangement, blocks fromdifferent units can be operated together as a metablock. A metablock,and consequently a zone, can cross multiple planes and each plane neednot be in the same physical device. The multiple dies can be operated inparallel independently of how they are actually packaged.

As with the memory embodiments described above, each block, the smallesterasable group of the memory array, is typically divided into multiplepages, a page containing the smallest number of cells that areprogrammable together within the block. Therefore, a programmingoperation of a metablock will usually include the simultaneouslyprogramming of data into at least one page of each of the blocks formingthe metablock, which is repeated until the metablock is full or theincoming data has all been programmed. Other metablocks are formed ofdifferent blocks from the array units, one block from each unit. Theextensions discussed below can all be combined with metablock operationand its extensions, such as those discussed in U.S. patent applicationSer. No. 10/081,375, filed Feb. 22, 2002, which is hereby incorporatedby reference.

FIG. 5 shows an exemplary embodiment with ten zones, each of which isdistributed across four planes. For example, Zone 0 is made up ofportions 620, 621, 622, and 623 that together make up the logicalstructure of a single zone. Bad blocks are shown in Zone 3, PLANE 1(601), in Zone 5, PLANE 2 (603), and in Zone 8, PLANE 0 (605). As shownin FIG. 5, the zone boundaries have been shifted between from plane toplane to accommodate these bad blocks. More generally, both the numberof planes and the number of zones can be a number of different values,although as discussed below there are various trade-offs involved in thechoice of these values.

The boundaries of each zone are selected so that at least a minimumnumber of good blocks are available for writing in each plane of thezone. Each zone should contain at least a number of blocks sufficient tohold data from blocks necessary for optimal system operation. Anyremaining spare blocks can be distributed evenly per zone per plane,where if the number of spares is not divisible by the number of planes,then the remaining extra blocks can be distributed evenly from the firstzone. The process is performed independently for each plane, with adiagram of the chip and how it is divided shown in FIG. 5.

In the exemplary embodiment, it is preferred that the zone boundaries donot cross device boundaries. This constraint is due to the preference inmemory for parallel operation in order to improve device performance, asdescribed above in the discussion of the related art. In otherembodiments, the planes can be distributed across multiple chips tofurther increase its flexibility and adaptability. For example, PLANE 0(620), PLANE 1 (621), and PLANE 2 (622) may be formed on one die andPLANE 3 (623) on another.

Every chip in a multi-chip device can be zoned in a similar fashion,with the exception that the first (or other) chip (chip 0) may havereserved system blocks and one or more spare blocks may be assigned tothe reserve area, thereby correspondingly reducing the number ofavailable spares for the user data portion of this chip. The uses of thereserved system blocks are discussed further below with respect toanother aspect of the present invention. There is no requirement thatBlock 0 be good. In fact, the firmware will tolerate bad blocks in thefirst 9 blocks of the chip.

The size of a zone is determined by a couple of factors. First, since alogical to physical translation table must be kept in the controllerRAM, an attempt is made to keep the zone size small for controller costreasons. Although dependent on the implementation, the present inventionwill generally need more area in RAM since in addition to storing therelation between zones and physical block numbers, it will now alsocontain information allowing zone size to vary from one plane to thenext. It also takes time to build these tables since a page form eachblock must be read to establish its logical address when a zone is firstaccessed. An opposing factor is that it is expanded as large as isreasonable to more efficiently use the extra physical blocks in theerase pool and allow for more defects.

For example, a particular embodiment is based on each memory chip havingfour planes with 4092 user data blocks plus 4 system blocks, for a totalof 4096. A set of sample calculations shows that both the amount of RAMto store a physical and the time to assemble this table decreaseinversely with the number of zones: Five zones takes twice the amountfor ten zones and three times the amount for fifteen zones. The defecttolerance also decreases in the same way. Consequently, a trade-offbetween these two effects must be chosen. For the described embodiment,a reasonable balance is to use an exemplary embodiment of 9 zones or the10 zones as shown in FIG. 5.

The process of adjusting the zone boundaries can be done at a number ofdifferent times. For example, it can be performed at the time of memorytest of the device to accommodate defective blocks in the fresh memoryarrays. In this case, a signal from external to the system is sent tothe controller, the bad blocks are determined and a zone tabledassembled. This table can then be stored in the reserved area of thenon-volatile memory for future use. This aspect of the present inventionimproves device yield. Previously, if there were too many defectiveblocks in a zone, the zone would be unusable. Now, the boundaries can beshifted to save this zone. Similarly, it can also increase device life.For example, if a zone had a maximal number of bad blocks at test time,it would previously become unusable at the first additional blockfailure, whereas now the extra good blocks are redistributed.

This aspect of the invention can also be implemented in a dynamic mannerto accommodate errors as they arise. This can be particularly useful forlate in life of system as blocks begin to fail. Although the spareblocks may be evenly distributed among zones after an initial boundaryadjustment at test time, new bad blocks will often not appear uniformlyacross a device. Non-uniform wear can result as one area cycles andwears more. There may also be a clustering of grown defects due tonon-uniform processing and the device will again run out of erase blocksin some zones.

The dynamic allocation of spare blocks can be motivated by severalsources. It may be done in response to a signal from the host based on,say, a time requirement. The host would issue a command to thecontroller that then goes ahead with the process. The controller itselfcould also initiate the process on its own, for example in response toan ECC result indicating a problem or to programming or erasedifficulties.

Whatever the impetus, the controller will determine the defect locationsin the memory array or arrays and determine the distribution of blocksto the zones to accommodate the defective blocks. As there may be datain a block that was formerly in one zone and is now to be reassigned toanother zone, the data in this block will need to be moved to keep it inthe same zone. If a number of defective blocks occur in a single zone,it will usually be easier on applications if the data movement andboundary reassignments are done in steps, for example moving boundariesby one block at a time. (When no data is present, such as at test time,the reassignment can all be done in one step.) This process is shownschematically in FIG. 6.

The boundary table is then updated by assembling the logical block tophysical block conversion for the new boundary assignments. This tablecan subsequently be moved from the controller RAM to a reserved portionof the non-volatile memory. By storing the zone table in non-volatilememory, when the system receives an initializing command from the host,the initializing the memory system will include transferring the zoneboundary table from the non-volatile memory to the controller's cachememory. The table can then be used to access the memory by zones.

FIG. 6 shows a portion of a plane with initial boundaries between zones8 and 9 and between zones 7 and 8 by the broken lines indicated by A. Inthe figure, zone 9 is shown to have 5 bad blocks, 701-705. For example,this could be a portion of the array where the memory cells were poorlyconstructed due to a processing error and which have eventually failedresulting in an insufficient number of good blocks in the zone. Severalgood blocks need to be transferred from zone 8 to zone 9 by moving theboundary to the location indicated by C. As this will reduce the numberof blocks in zone 8, a further adjustment may be need to transfer someblocks form zone 7 to zone 8 by moving that boundary to the locationindicated there by B.

As the blocks indicated by 711 and 712 are likely contain data, thisdata have to be moved if their logical block numbers are to remain inthe same zone and preserve the zone structure. As there may not beenough available space to move both blocks at once, the block 711 can bemoved first and an intermediate boundary established between zones 8 and9 as indicated there by B. To create additional room in zone 8 and allowits boundary with zone 7 to be moved from A to B, the data in block 721needs to be similarly moved. Subsequently to moving the data in block721 to zone 7 and establishing the boundary indicated there by B, thedata in block 712 can be moved to zone 8 and the C boundary established.

As for the relative timings, if the additional space created by movingblock 721 is need to move block 712, it is only necessary that block 721is moved sometime before block 712. This could be after, at the sametime as, or even prior to moving block 711. If there is no data in theseblocks, for example at the time of testing, there is of course no datato move and the final boundaries can be established without intermediatesteps.

FIG. 7 is a flowchart of the process. The process can be initiated in anumber of ways. It can be start by an external signal (801), for exampleat test time or as decided by a host. The host can initiate the processbased upon timing or number of operations, for example a number oferase/program operations. This sort of initiation can also originatewith the controller itself. Another way the process can be initiatedwithin the system itself is in response to an indication of defects inpart of the array (802), for example due to an ECC result or failure toverify in a program or erase process.

The process itself begins with the controller evaluating the selectedportions of the memory (803) to determine which, if any, blocks aredefective. Based on this result, the controller determines thedistribution of good blocks that will distribute the good blocks in aplane across the zones, as described above. This equalized the number ofgood blocks—or, more accurately minimizes the variation in the number ofgood blocks—as described above in order to produce the zone table (805).In the exemplary embodiment, a firmware implementation is used in thecontroller, although implementations using only hardware or software arepossible variations. In a further set of variations, some or all of theboundary determination can be performed in the host.

As described with respect to FIG. 6, it may be necessary to move datathat will be affected by boundary changes. This is done in step 807,after which the boundary is adjusted in step 809. As also describedabove with respect to FIG. 6, this process may be done in an incrementalprocess (811). When no data is present, such at initial device testing,these steps are not needed.

At this point, the zone table is determined and the boundaries set ineach plane and the actual process is done. The device can now beoperated using the zone table to access the memory (813). Eventually,the process may be repeated based on the result of accessing the memory,as described with respect to step 802. Also, the updated zone table canalso be stored in non-volatile memory (815), for example in the reservedportion, to maintain this information for future use. Although step 815is shown in FIG. 7 to occur after step 813, it can be done any timeafter the table is established.

As mentioned above, although the discussion so far has referred mainlyto embodiments using a charge storing device, such as floating gateEEPROM or FLASH cells, for the memory device, it can be applied to otherembodiments, including magnetic and optical media. As the particulars ofhow the storage elements are read, are written, and store data do notenter into the main aspects of the present invention, the variousaspects of the present invention may be applied to other memory types,including, but not limited to, sub 0.1 μm transistors, single electrontransistors, organic/carbon based nano-transistors, and moleculartransistors. For example, NROM and MNOS cells, such as thoserespectively described in U.S. Pat. No. 5,768,192 of Eitan and U.S. Pat.No. 4,630,086 of Sato et al., or magnetic RAM and FRAM cells, such asthose respectively described in U.S. Pat. No. 5,991,193 of Gallagher etal. and U.S. Pat. No. 5,892,706 of Shimizu et al., all of which arehereby incorporated herein by this reference, could also be used.

Although the invention has been described with respect to variousexemplary embodiments, it will be understood that the invention isentitled to protection within the full the appended claims.

It is claimed:
 1. A non-volatile memory system, comprising: a memorycircuit, including: one or more physical memory arrays, each of said oneor more physical memory arrays having a plurality of non-volatilestorage elements; programming circuitry selectable connectable to thestorage elements to write data content thereto; read circuitryconnectable to the storage elements to read data content stored therein;and erase circuitry connectable to the storage elements to erase datacontent stored therein, the one or more physical memory arrays eachbeing formed of a plurality of physical blocks, wherein the storageelements within individual ones of the physical blocks aresimultaneously erasable; and a memory controller circuit that controlswriting of data into addressed blocks, reading data from addressedblocks and erasing data from one or more of addressed blocks at a time,wherein the one or more physical memory arrays are subdivided by thecontroller into logical zones each comprised of a plurality of physicalblocks by introducing logical boundaries of the one or more physicalmemory arrays for address translation within which logical to physicalblock translations are performed for the plurality of physical blocksthereof, and where the logical zones correspond to contiguous physicalspaces by introducing beginning and end physical block addresses for thelogical zones, wherein the memory controller circuit can adjust saidlogical boundaries and update said correspondence in response toidentifying one or more of said physical blocks as defective based onthe distribution of the identified defective physical blocks whilemaintaining a correspondence of physical blocks to logical zonescorresponding to contiguous physical spaces.
 2. The memory system ofclaim 1, wherein the identifying of physical blocks as defective isbased on testing data.
 3. The memory system of claim 2, wherein thememory controller circuit performs testing of the physical blocks inresponse to a signal external to said memory system.
 4. The memorysystem of claim 2, wherein said memory controller circuit furthercomprises error correction code (ECC) logic and performs testing of thephysical blocks in response to an error correction code result.
 5. Thememory system of claim 1, wherein the memory controller circuitidentifies a physical block as defective based on a program verificationresult.
 6. The memory system of claim 1, wherein the memory controllercircuit identifies a physical block as defective based on an eraseverification result.
 7. The memory system of claim 1, wherein the memorycontroller circuit updates said correspondence based on minimizing thevariation in the number of non-defective physical blocks in said logicalzones.
 8. The memory system of claim 1, wherein the memory controllercircuit further includes a volatile memory in which said correspondenceis maintained.
 9. The memory system of claim 8, wherein the memorycontroller circuit additionally maintains a copy of said correspondencein non-volatile memory.
 10. The memory system of claim 1, wherein inresponse to updating said correspondence, the memory system rewritesuser data stored in a first physical block into a second physical block,wherein said first physical block is part of a first logical zone priorto said updating and is part of second logical zone subsequent to saidupdating, and wherein said second physical block is part of said firstzone subsequent to said updating.
 11. The memory system of claim 10,wherein subsequent to rewriting user data stored in a first physicalblock into a second physical block, the memory controller circuitfurther updates said correspondence on the distribution of theidentified defective blocks.